Issue A&A Volume 450, Number 2, May I 2006 Page(s) 625 - 643 Section Interstellar and circumstellar matter DOI http://dx.doi.org/10.1051/0004-6361:20053417

A&A 450, 625-643 (2006)
DOI: 10.1051/0004-6361:20053417

Radio continuum and molecular line observations of four bright-rimmed clouds

J. S. Urquhart¹, M. A. Thompson², L. K. Morgan^{3, 4} and G. J. White<sup>5, 6

1</sup>  Department of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
    e-mail: jsu@ast.leeds.ac.uk
²  School of Physics Astronomy & Maths, University of Hertfordshire, College Lane, Hatfield, AL10 9AB, UK
³  Centre for Astrophysics and Planetary Science, School of Physical Sciences, University of Kent, Canterbury, CT2 7NR, UK
⁴  Green Bank Telescope, PO Box 2, Green Bank, WV 24944, USA
⁵  Dept. of Physics & Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
⁶  Space Physics Division, Space Science & Technology Division, CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK

(Received 12 May 2005 / Accepted 15 January 2006 )

Abstract
Aims.To search for evidence of triggered star formation within four bright-rimmed clouds, SFO 58, SFO 68, SFO 75 and SFO 76.
Methods.We present the results of radio continuum and molecular line observations conducted using the Mopra millimetre-wave telescope and Australia Telescope Compact Array. We use the J=1-0 transitions of ¹²CO, ¹³CO and C¹⁸O to trace the distribution of molecular material and to study its kinematics. The radio continuum data is used to trace the distribution of the ionised gas and to derive its parameters. Combining these observations with archival data allows us to build up a comprehensive picture of the current state of star formation within these clouds.
Results.These observations reveal the presence of a dense core (  cm^-3) embedded within each cloud, and the presence of a layer of hot ionised gas coincided with their bright-rims. The ionised gas has electron densities significantly higher than the critical density (>25 cm^-3) above which an ionised boundary layer can form and be maintained, strongly supporting the hypothesis that these clouds are being photoionised by the nearby OB star(s). Using a simple pressure-based argument, photoionisation is shown to have a profound effect on the stability of these cores, leaving SFO 58 and SFO 68 on the edge of gravitational stability, and is also likely to have rendered SFO 75 and SFO 76 unstable to gravitational collapse. From an evaluation of the pressure balance between the ionised and molecular gas, SFO 58 and SFO 68 are identified as being in a post-pressure balance state, while SFO 75 and SFO 76 are more likely to be in a pre-pressure balance state. We find secondary evidence for the presence of ongoing star formation within SFO 58 and SFO 68, such as molecular outflows, OH, H₂O and methanol masers, and identify a potential embedded UC HII region, but find no evidence for any ongoing star formation within SFO 75 and SFO 76.
Conclusions.Our results are consistent with the star formation within SFO 58 and SFO 68 having been triggered by the radiatively driven implosion of these clouds.

Key words: stars: formation -- ISM: clouds -- ISM: HII regions -- ISM: general -- ISM: molecules -- radio continuum: ISM

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